As a method for realizing high performance of an optical disc apparatus, signal processing of recording pits formed on a track on a disc is simultaneously performed, in parallel, for a plurality of tracks, and the processing speed is increased. In order to realize the same, development of an LD chip including a plurality of light emitting points which are separated from each other by separating grooves and independently driven in a single LD chip is advancing.
In addition, in order to realize a system in which immediately after a signal is recorded, the signal is reproduced and error checking is performed, it is necessary to constitute a two-point array in which a recording laser diode and a reproduction laser diode are arranged at two points in a line and they are controlled independently.
Conventionally, an LD chip provided with a linear array of multiple light emitting points with an interval of 50-100 .mu.m on a same semiconductor substrate has been reported. In this system, however, Joule heat generated at an LD is conducted to an adjacent LD via the semiconductor substrate, whereby the light output is reduced, which phenomenon is called thermal crosstalk.
A solution to this problem is described, for example, in an article of "Hybrid laser array with closely spaced dual beams" in 35-th Applied Physics of Japan and Related Societies Meeting (1988 spring) Lecture Prescript pp. 898, 30p-ZQ-3.
This laser array with closely spaced dual beams has a structure in which two LD chips each having a single light emitting point are disposed on individual substrates functioning as a cooler and arranged with the light emitting points close to each other, in order to prevent thermal crosstalk.
FIG. 13 is a perspective view of this prior art laser array with closely spaced dual beams.
In FIG. 13, reference numeral 1 designates a stem. A high output laser diode (hereinafter referred to as high output LD) block for recording 2 is disposed on the stem 1. A high output LD submount 3 is mounted on the high output LD block 2. A high output LD chip 4 is mounted on the high output LD submount 3. A high output LD feeding wire 5 is connected to an electrode on the surface of the high output LD chip 4. Reference numeral 6 designates a light emitting point of the high output LD. A low noise LD block for reproduction 7 is disposed on the stem 1. A low noise LD submount 8 is mounted on the low noise LD block for reproduction 7. A low noise LD feeding wire 10 is connected to an electrode on the surface of the low noise LD chip 9. Reference numeral 11 designates a light emitting point of the low noise LD. Reference numeral 12 designates a lead.
In the laser array with closely spaced dual beams constructed as described above, the high output LD chip 4 and the low noise LD chip 9 are separately fixed to the high output LD block 2 and the low noise LD block 7 via the high output LD submount 3 and the low noise LD submount 8, respectively, and the LD chips 4 and 9 are fixed to the stem 1 by the high output LD block 2 and the low noise LD block 7, respectively, so that the light emitting point 6 of the single high output LD and the light emitting point 11 of the single low noise LD are opposed to each other with an interval of about 20 .mu.m. Accordingly, the light emitting point 6 of the high output LD and the light emitting point 11 of the low noise LD can be spatially separated, thereby producing an advantage that thermal interference is not likely to occur.
In the laser array with closely spaced dual beams constructed as described above, when the temperature variation of the environment in which the LD array is employed is small, the above-described construction is thought to be sufficient. However, when the temperature variation of the environment is larger, the stem 1, the high output LD block 2, and the low noise LD block 7 change thermally and the interval between the light emitting point 6 of the high output LD and the light emitting point 11 of the low noise LD varies. For example, when the LD array is applied to an optical disc apparatus, the interval between the two light emitting points expands and contracts and, the deviation of the LD array from the optical system in the optical disc apparatus cannot be adjusted sufficiently only by the positional adjustment using the parallel movement of a semiconductor laser device to which the LD array is mounted. This presents preferable recording and reproduction.